1. Field of the Invention
The present invention relates to an equalizer circuit for use in a communication unit. More particularly, the present invention relates to an equalizer circuit for use in a TDMA communication system, for example, and for equalizing an amplitude distortion and/or group delay distortion caused in a transmit-receive system.
2. Description of the Prior Art
FIG. 1 is a conceptual drawing showing an example of a TDMA communication which is a background of the present invention. The TDMA communication is used for a satellite communication, for example, which comprises a plurality of earth stations ES, ES', . . . and a common communication satellite CS. The earth station ES comprises a transmitting apparatus TRA and a receiving apparatus REA. A signal modulated by a modulator MOD included in the transmitting apparatus TRA is sent through an equalizer EQL, a transmitter TR and an antenna AE toward an antenna AS of the communication satellite CS. The signal is frequency converted and the frequency converted signal is sent to another earth station ES'. Similarly, a signal from the other earth station ES' is received by the antenna AE of the earth station ES through the communication satellite CS and the received signal is provided to the receiving apparatus REA. In the receiving apparatus REA, the received signal passes a receiver RE and an equalizer EQL and is demodulated by a demodulator DEM. It is known that the transmitter TR and the receiver RE in the earth station ES and a receiving and transmitting system in the communication satellite cause an amplitude distortion and/or group delay distortion, respectively. Particularly, a high power amplifier (not shown) included in the communication satellite CS is used in a relatively saturated state due to size, price and stability thereof. As a result, an AM-PM conversion is caused in the high power amplifier, which causes a phase variation as shown in line A in FIG. 2 in which the line B denotes an output level. Such a phase variation becomes a group delay distortion.
The amplitude distortion and group delay distortion are, respectively, equalized for an amplitude frequency response and a group delay frequency response by the equalizer EQL included in the transmitting apparatus TRA and the equalizer EQL included in the receiving apparatus REA. Conventionally, such an equalizer EQL is generally structured to include a fixed amplitude equalizer FAE, a fixed group delay equalizer FDE and a variable equalizer ME, as shown in FIG. 3. Either of the fixed amplitude equalizer FAE and the fixed group delay equalizer FDE or both may be omitted depending on the amount of actual amplitude distortion or group delay distortion.
In the TDMA communication system which is a background of the present invention, it is impossible that once an operation is initiated, the above described amplitude frequency response and group delay frequency response are detected by transmitting and receiving test signals so that an optimum amount of equalization is detected. The reason is that the time period when a single earth station occupies a line is extremely short since such communication system is made in a time divisional manner. Therefore, in case where a new earth station joins such a communication satellite system, it is necessary to seek an optimum point where amplitude distortion and group delay distortion are minimum and thus a bit error rate (BER) is minimum. To this end, a variable equalizer ME as shown in FIG. 3 may be used.
FIG. 4 is a circuit diagram showing an example of a conventional variable equalizer which is a background of the present invention. An input signal inputted to an input terminal 1 branches through a branch circuit 2, a portion of which being applied to an attenuation setting circuit 4 having a coefficient -a.sub.n and the remaining signal being inputted to the next branch circuit 2 through a delay line 3 having a delay amount T. In a similar operation, the respective signals are applied to the respective attenuation setting circuits having respective attenuation amounts. The signals, from the attenuation setting circuits 4, 4, . . . are all applied to an adder 5 and thus the synthesized signals are outputted from the adder 5 to an output terminal 6. The attenuation setting circuits 4, 4, . . . include polarity reverse. In such a way, the attenuation amounts of the attenuation setting circuits 4, 4, . . . are set to a.sub.0 =1 at the center, +a.sub.1 and -a.sub.1 on the two sides thereof having absolute values which are equal but polarities which are opposite to each other, +a.sub.2 and -a.sub.2, . . . , +a.sub.n and -a.sub.n. In such a manner, an amplitude frequency response and group delay frequency response are set through a known transversal filter theory by arbitrarily setting the respective attenuation amounts of the attenuation setting circuits 4, 4, . . . . More particularly, an optimum point is sought by the variable equalizer ME by varying an amplitude frequency response and group delay frequency response while detecting a bit error rate (BER). Such a transversal filter is described in a book entitled "Data Transmission", William R. Bennet and James R. Davey, published by McGraw-Hill Book Co., 1965, for example.
In the TDMA communication system, the bit error rate is more greatly affected by group delay distortion than by amplitude distortion and thus an operation for seeking such an optimum point is easily made if an optimum equalization amount for the group delay distortion can be set. Nevertheless, a conventional variable equalizer has not been able to vary, for example, only the amplitude or only the group delay since the attenuation amounts of the attenuation setting circuits 4, 4, . . . are arbitrarily set. Accordingly, this means that it is difficult to seek an optimum point by a conventional variable equalizer in a TDMA communication system in which the influence of the group delay frequency response is greater than that of the amplitude frequency response. In addition, although attenuation amounts of a conventional variable equalizer determine an amplitude frequency response and group delay frequency response, respectively, how such frequency response characteristics vary when a single attenuation amount varies could not be known without a large number of simulation data since such variation is different depending on other coefficients. For this reason, it can not be easily confirmed how the amplitude and group delay are equalized.
Another variable equalizer as shown in FIG. 5 has been previously proposed by the present applicant, for example. In FIG. 5, an input signal applied from an input terminal 1 is distributed by a distributor 7. The signal distributor 7 distributes a signal into three signals of the same level. A delay line 3 having a delay amount T is interposed in one path for one of the three signals, a delay line 31 having a delay amount 2T is interposed in another path, and a polarity inverter 8 is interposed in the remaining path. The polarity inverter 8 is structured by a known transformer or transistor and the like and shifts a phase of an applied signal by 180.degree. . The signal from the delay line 31 and the signal from the polarity inverter 8 are synthesized by an adder 9 to be applied to a variable attenuation setting circuit 10. The variable attenuation setting circuit 10 comprises a polarity reverse and an output signal therefrom is synthesized with an output signal from the delay line 3 by an adder 11.
Let it be assumed that no attenuation of signal is caused except for the variable attenuation setting circuit 10 and that no time delay is caused except for the delay lines 3 and 31 and the delay of a main signal is used as a reference (zero). Under these assumptions, an output signal B(.omega.) obtained from the output terminal 6 is represented in the following equation (1). ##EQU1## The response characteristic G.sub.B (.omega.) of the amplitude with respect to the frequency of the output signal B(.omega.) and the response characteristic .tau..sub.B (.omega.) of the delay amount with respect to the frequency of the output signal B(.omega.) are provided by the following equations (2) and (3), respectively. ##EQU2## In the equations, .omega. is an angular frequency, .omega.=2.pi.f wherein f is a frequency. Variation of the amplitude frequency response characteristic G.sub.B (.omega.) and the group delay frequency response characteristic .tau..sub.B (.omega.) where a coefficient l is larger than 0 (l&gt;0), are shown in the FIG. 6. FIG. 6(A) shows an amplitude frequency response and FIG. 6(B) shows a group delay frequency response, wherein the amplitude and delay amount varies in the direction of the arrow when the coefficient l is made larger. More particularly, as shown in FIG. 6, in the FIG. 5 example, the delay amount varies if the coefficient l is varied in the attenuation setting circuit 10. However, even in the FIG. 5 example, the amplitude as well as the delay amount is varied according to the variation of the coefficient l and thus it is extremely difficult to utilize the FIG. 5 example as a variable equalizer in the TDMA communication system.